Recent activity in oil field exploration in response to an increased demand for the exploitation of additional sources of petroleum beyond those currently known and used has led in the establishment of oil wells in remote locations. This has resulted in a need for more versatile and efficient equipment for use at a well head which may be easily and conveniently serviced. A primary deficiency of many currently known pumping and drilling units for oil field use is the fact that such units may not be rapidly and conveniently serviced with replacable components which permit the unit to be placed back into operation after a minimum "down time".
One extremely important assembly used during the drilling of wells by the rotary method is a mud pump for circulating drilling mud downwardly to the drill bit and upwardly outside the drill pipe to the surface. Mud pumps are required to provide drilling mud under relatively high pressures and volume to handle fluid containing a considerable amount of abrasive material. This combination results in a high degree of pump wear which is often accentuated by the design of existing pumps and pump drives. For example, known gear driven mud pump units experience excessive gear wear in the drive between a driving engine and the pump which necessitates frequent replacement of the gear drive, a maintenance feature which is both time consuming and expensive. Such gear driven units, as well as prior known hydraulic units, employ a very short stroke with a high piston speed in order to provide the required volume of pressurized drilling mud, and the resulting passage of abrasive fluid through the pump cylinders causes the rapid abrasion of cylinder liners. Also, with short stroke pumps, rapid piston reversal causes pounding of the drilling mud control valves as well as an extremely high number of valve actuations per unit of time. These factors all result in excessive wear and high maintenance requirements which are not acceptable at remote drilling locations.
In the past, it has been recognized that the deficiencies of existing drilling mud pumps could be substantially rectified by the development of a long stroke hydraulic pump which would provide smooth operation and relatively few reversals per minute. However, with a single cylinder pump, the long piston rods required to provide a long, slow stroke have a tendency to buckle under compression, and this has inhibited the development of such pumps. In an attempt to develop a slower stroke pump and eliminate rapid valve actuation, coaxial cylinder pumps with a single piston rod and dual pistons have been developed wherein motive fluid is centrally applied to the coaxial piston structure. This motive fluid is selectively supplied to either one pump cylinder or another to control the direction of piston movement. A coaxial pump system of this type is disclosed by U.S. Pat. No. 2,283,207 to A. Hollander, and although such units do provide a slower stroke pumping action than conventional short stroke units, these units are not designed for effective component replacement, nor do they provide a high volume, high pressure output with a minimal horse power input. Also the pump action and the pump drive are occurring in the same cylinders which is undesirable.
Consequently, a need exists for an effective drill mud pumping unit which may be easily shipped and transported to remote oil fields, readily and quickly errected at a well head by a minimal work crew to form a very stable supporting structure for the pumping apparatus, and which includes features whereby the unit will function at optimum efficiency with a low stroke speed and high volume.